1. Field of Use
The present invention relates to an armature group for a mosaic printing head and related manufacturing method.
2. Prior Art
A mosaic printing head comprises a plurality of printing needles and a corresponding plurality of actuating electromagnets radially arranged on a bearing ring. Each electromagnet is provided with a movable armature having one end extending beyond the magnetic circuit of the electromagnet which acts as thrust arm for one of the printing needles. The contact points between each of the movable armatures and the related needle heads are uniformly arranged along a circumference located on a plane parallel to the electromagnet bearing ring. A distribution of the needle printing ends according to one or more parallel columns is obtained by having the needles elastically bent so as to assume a gradual bending controlled and supported by suitable guides.
A retainer for the several electromagnets armatures is associated with the electromagnetic group which comprises the plurality of electromagnets. Such armature retainer, besides enabling correct armature movement and defining the width of the air gap of the several electromagnets in rest position, further acts as damper when an electromagnet armature changes from an actuated state to a release state. Examples of such heads are disclosed in U.S. Pat. Nos. 4,260,270 and 4,367,962.
Low cost and high reliability are particularly important for a mosaic printing head. The manual assembling time is a factor which greatly affects the cost of a mosaic printing head. Several head manufacturers try to reduce to a minimum, the number of parts of a head which are manually assembled so as to cut the assembling time, such parts being produced by automated processes. U.S. Pat. No. 4,433,927 which is assigned to the applicant discloses, for example, an electromagnetic group for a mosaic printing head and an automated process for producing such group as a unitary piece.
At present, the most critical phase in the head assembly is that of separately mounting the armatures in suitable positions on the magnetic circuits. In fact, besides the relatively long time required by such operation, it is difficult to subsequently mount the armature retainer without affecting the positions of these armatures, particularly in the case of heads having a large number of armatures, such as 14 or 18 needle heads.
U.S. Pat. No. 4,140,406 suggests that the retainer include a series of projections for sustaining and guiding each armature. During assembling, each of the armatures is inserted between such projections and held in position therein. Subsequently, the retainer, together with the armatures, can be assembled with the electromagnet group.
Such solution, besides involving a retainer which is complex to construct, still requires the manual insertion of armatures into the appropriate retainer housings. Further, the projections can apply variable amounts of friction to the armatures during the printing head operation, causing the non-uniform behavior of several printing elements.
The reliability of the mosaic heads largely depends upon the breaks that a printing needle may undergo due to the stresses applied during the actuation phase by the corresponding armature. The generalized use of a flat armature does not allow for correct contact between the needle head and the armature. Thus, during the actuation phase, an undesired moment is generated on the guides and the needle head. Due to such moment, the needle may undergo a buckling force which can reach the breakage limit.
In order to overcome such disadvantage, previously mentioned U.S. Pat. No. 4,140,406 suggests bending the end of the armature protruding outside the magnetic circuit of the electromagnet, in order that such end be perpendicular to the needle axis in correspondence with the contact point. However, such solution does not completely eliminate the undesired moment on the guides and on the needle head so that from this point of view, the head has little reliability.
A first object of the present invention is to reduce the head cost by cutting to a minimum, the manual assembly time of the armature on the magnetic circuits by simplifying the structure of such armature retainer.
A further object of the present invention is to increase the head reliability by almost completely eliminating the undesired moment on the guides and on the needle head during the actuation phase.